The present invention relates to a touch panel device for detecting the contact of an object such as a finger or a pen with the touch panel device, and more particularly relates to a touch panel device for detecting the contact position of the object by detecting attenuation and cutoff of surface acoustic waves (SAWs) by using excitation elements and receiving elements, each constructed by forming electrodes on a piezoelectric body.
With the spread of computer systems, mainly personal computers, there has been used a device for inputting new information or giving various instructions to a computer system by pointing at a position on a display screen of a display device on which information is displayed by the computer system, with an object such as a finger or a pen. In order to perform an input operation with respect to the information displayed on the display screen of the display device of a personal computer or the like by a touching method, it is necessary to detect the contact position (pointed position) on the display screen with high accuracy.
Well known examples of a touch panel device for detecting the contact position of an object such as a finger and a pen are a device using a resistance film, and a device using ultrasonic waves. The former device using a resistance film detects a change in the resistance of the resistance film caused by contact of the object with the resistance film. This device has the advantage of low consumption of power, but has problems in the aspects of the response time, detection performance and durability.
By contrast, in the device using ultrasonic waves, the contact position of the object is detected by propagating surface acoustic waves on a non-piezoelectric substrate, for example, and detecting attenuation of the surface acoustic waves caused by contact of the object such as a finger and a pen with the non-piezoelectric substrate. There has been developed a touch panel device that uses, as transducers for exciting and receiving surface acoustic waves, comb-like electrodes (IDTs: inter digital transducers) that can be produced collectively using a photolithography technique. In this touch panel device, as each of excitation elements for exciting surface acoustic waves and receiving elements for receiving propagated surface acoustic waves, an element constructed by forming a comb-like electrode on a piezoelectric body in the form of a thin film is used.
FIG. 1 is an illustration showing the configuration of such a conventional touch panel device using comb-like electrodes. In FIG. 1, the numeral 61 represents a rectangular non-piezoelectric substrate. A plurality of excitation elements 62, each constructed by forming a comb-like electrode on a piezoelectric body, for exciting surface acoustic waves are arranged in a line on one end of each of the X-direction and Y-direction of the non-piezoelectric substrate 61. Moreover, a plurality of receiving elements 63, each constructed by forming a comb-like electrode on a piezoelectric thin film, for receiving surface acoustic waves are arranged in a line on the other end of each of the X-direction and Y-direction of the non-piezoelectric substrate 61.
In this touch panel device, periodic signals are inputted to the excitation elements 62 to excite surface acoustic waves and propagate them on the non-piezoelectric substrate 61, and then the propagated surface acoustic waves are received by the receiving elements 63. Then, when an object such as a finger and a pen comes into contact with the propagation path of the surface acoustic waves on the non-piezoelectric substrate 61, the surface acoustic waves attenuate. Accordingly, it is possible to detect the presence or absence of contact of the object and the contact position by detecting whether or not the level of the received signals at the receiving elements 63 is attenuated.
In the touch panel device with such a structure, since the resolution performance is determined by the aperture length of the comb-like electrode, a small aperture length is preferable. However, there is a correlation between the aperture length and the wavelength of the surface acoustic wave, and, when the aperture is narrowed, the diffraction effect is increased abruptly and it becomes difficult to distinguish a path on which the surface acoustic wave is propagated. Hence, in order to improve the resolution performance for the detection position, the present inventor et al. have proposed a touch panel device in which the excitation elements and receiving elements are disposed so as to propagate surface acoustic waves in oblique directions (diagonal directions) of the non-piezoelectric substrate.
FIG. 2 is an illustration showing the configuration of an example of such a touch panel device (hereinafter referred to as the conventional example). In FIG. 2, the numeral 71 represents a rectangular non-piezoelectric substrate, and a center portion enclosed by an alternate long and short dashed line is a detection region 71a capable of detecting the contact position.
In a frame region 71b outside the detection region 71a, which is a peripheral section of the non-piezoelectric substrate 71, excitation elements 72 are disposed on the upper side and lower side of the substrate 71, while receiving elements 73 are positioned on the left side and right side thereof. FIG. 3 is a partial cross sectional view of the excitation element 72 or the receiving element 73, and the excitation element 72 or the receiving element 73 is constructed by forming a comb-like electrode 75 on a piezoelectric body 74 in the form of a thin film. This comb-like electrode 75 comprises facing bus electrodes 77 and a plurality of electrode fingers 78 which are extended from the bus electrodes 77 by turns and bent in the middle. In this structure, lines of a plurality of electrode fingers 78 tilted in two directions from the facing direction of the bus electrodes 77 are formed, thereby realizing simultaneous excitation of surface acoustic waves in two directions and simultaneous reception of surface acoustic waves from two directions. Furthermore, in the frame region 71b, there are provided drawn-round wires 79 for connecting the bus electrodes 77 to external circuits (such as an oscillation circuit and a receiving level detecting circuit).
In such a structure, surface acoustic waves are excited in two directions by the excitation elements 72, and the excited surface acoustic waves are propagated in two diagonal directions of the non-piezoelectric substrate 71 and then received by the receiving elements 73. Based on the received results, the presence or absence of contact of an object and the contact position are detected in the same manner as in the conventional example shown in FIG. 1.
In the touch panel device, since the frame region outside the detection region, in which the excitation elements, receiving elements, drawn-round wires, etc. are placed, is a region where the contact position of an object is not detectable, it is necessary to narrow the area of this frame region. Since the above-described conventional example has a pattern of electrode fingers with different polarities which are alternately present (see FIG. 3), it is necessary to provide a bus electrode for each polarity. Accordingly, each side needs to have a width for two lines of bus electrodes in addition to the width of the excitation/receiving region (region for forming the electrode fingers), and consequently there is the problem of an increase in the area of the frame region.
Furthermore, in each of the boundary areas (four corners) between a side where the excitation element is placed and a side where the receiving element is placed, a width for the bus electrodes is also necessary. Therefore, a region (a section near the diagonal line shown by hatching in FIG. 2) where the surface acoustic waves propagated in the diagonal direction are weak becomes wider, and the touch panel device may suffer from the problem of a decrease in the detection accuracy.